The instant invention relates generally to processes for the preparation of parylene dimers, and more particularly to processes for the preparation of octafluoro-[2,2]paracyclophane, otherwise known as AF4.
Parylene is a generic term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor for, and fed to a vacuum chamber wherein the monomer molecules polymerize, and deposit onto a substrate disposed within the vacuum chamber.
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene materials are ideally suited for use as a conformal coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (AF4) is a fluorine substituted version of the above-noted dimers and has the structure: ##STR3## It is known that parylene coatings (Parylene AF.sub.4) which are derived from the AF.sub.4 dimer by the vapor deposition process have a very high melting temperature (about 540.degree. C.), and a low dielectric constant (about 2.3). These characteristics make Parylene AF.sub.4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material for the production of semiconductor chips. However, up to the present time, AF4,which is used as the dimer starting material for depositing Parylene F coatings, has been commercially unavailable due to high costs of production.
One known method of producing AF4 is described in U.S. Pat. No. 5,210,341 wherein the process of preparing AF4 utilizes a low temperature in conjunction with a reduced form of titanium in order to produce dimerization of dihalide monomers. One aspect of the '341 patent provides a process for preparing octafluoro-[2,2]paracyclophane, which comprises contacting a dihalo-tetrafluoro-p-xylylene with an effective amount of a reducing agent comprising a reduced form of titanium and an organic solvent at conditions effective to promote the formation of a reaction product comprising octafluoro-[2,2]paracyclophane.
While the process described in the '341 patent is effective for its intended purpose, it has been found that the process is still too expensive for commercial realization due to low yields, that there are some impurities in the AF4 dimer, and furthermore that it would be difficult to adapt to a large scale commercial production.
The instant invention provides improved processes for the preparation of octafluoro-[2,2]paracyclophane which involve contacting a reactant comprising 1,4-bis-(bromodifluoromethyl)benzene with a reducing agent comprising trimethylsilyltributyltin (TMSTBT), and fluoride ions in a refluxing solution of hexamethylphosphoramide (HMPA) or dimethylsulfoxide (DMSO) in tetrahydrofuran (THF) at conditions effective to promote the formation of a reaction product comprising octafluoro-[2,2]paracyclophane.
The dibromide reactant in the present invention is the reactant which is most efficiently converted to AF4. Alternatively, the analogous dichloride [1,4-bis(chlorodifluoromethyl)benzene], or diiodide [1,4-bis(iododifluoromethyl)benzene] may be used. However, the obtained yield of AF4 appears to be lower when using them in place of the dibromide.
In one preferred aspect of the invention, anhydrous tetrahydrofuran (THF), hexamethylphosphoramide (HMPA), and anhydrous cesium fluoride (CsF) are added under a nitrogen atmosphere to a flask provided with a mechanical stirrer. After initial warming and stirring, a quantity of trimethylsilyltributyltin (TMSTBT) is added to the mixture. After further stirring a predetermined quantity of 1,4-bis(bromodifluoromethyl)benzene (otherwise referred to as dibromide) is added to the mixture. Over a period of up to 24 hours, additional quantities of CsF, TMSTBT, and dibromide are added to the mixture. After all of the dibromide is consumed by the formation reaction, the THF is removed by distillation, and evaporation, and then various other filtering, distillation, evaporation, and washing steps are conducted to yield a crystalline form of AF4 (8-14% yield).
In another preferred aspect of the invention, the HMPA in the above process is replaced by dimethylsulfoxide (DMSO). The later steps are substantially as described above, yielding a crystalline form of AF4. However, the yield is somewhat higher with the DMSO process (up to 40% yield).
Accordingly, among the objects of the instant invention are: the provision of improved processes for the preparation of octafluoro-[2,2]paracyclophane; and more specifically, the provision of improved processes for the preparation of octafluoro-[2,2]paracyclophane which involves contacting a reactant selected from the group consisting of 1,4-bis(bromodifluoromethyl)benzene, 1,4-bis(chlorodifluoromethyl)benzene, and 1,4-bis (iododifluoromethyl)benzene with a reducing agent comprising trimethylsilyltributyltin (TMSTBT), and fluoride ions in a refluxing solution of hexamethylphosphoramide (HMPA) or dimethylsulfoxide (DMSO) in tetrahydrofuran (THF) at conditions effective to promote the formation of a reaction product comprising octafluoro-[2,2]paracyclophane.
Other objects, features and advantages of the invention shall become apparent as the detailed description thereof proceeds.